1. Field of the Invention
This invention relates to switching regulator power supplies. More specifically, this invention relates to systems which provide overcurrent protection for such power supplies.
While the present invention is described herein with reference to a particular embodiment in a particular application it is to be understood that the invention is not limited thereto. Those having ordinary skill in the art to which this invention pertains will recognize modifications and other applications within the scope thereof.
2. Description of the Prior Art
As discussed by Abraham I. Pressman in Switching and Linear Power Supply Design, published by the Hayden Book Company in 1977, pages 183 through 185, the routine testing and maintenance of power supplies is such that the output of power supply may be accidentally shorted to ground. Overload currents may occur from a breakdown or malfunction in the load circuit. While most overload currents are usually temporary, it is desirable that they do not damage the power supply. For this reason, overcurrent protection circuits have been employed with some measure of success.
Prior art overcurrent protection circuits are primarily intended to protect against over dissipating linear mode series pass elements when an output node is shorted to ground. Although the prime reason for prior art overcurrent protection is to protect against such dead shorts at the output of the power supply, it is also useful to protect against smaller limited overcurrents above the maximum specified load current.
Pressman discusses two generally used modes of overcurrent protection: constant current and current foldback. In a constant current protection system, after output current is increased up to the maximum specified overload current, the output current is held constant. In a current foldback system, the output voltage remains constant and within specification limits up to a predetermined current level. Beyond this current level, the output voltage and the current start falling back along a "foldback" line.
For linear series pass regulators the current foldback technique may have advantages over the constant current technique. However, neither is directly applicable to a switching regulator type power supply. Prior art overcurrent limiters for switching regulators are adaptations of those previously developed for linear regulators. As such the prior art switching regulator overcurrent limiter circuits depend on the resetting of a switching inductor core. This may be costly and otherwise difficult to achieve.
In addition, hysterisis type switching regulators (non-clocked) using prior art overcurrent limiters, have a switching frequency that is related to the magnitude of the overload resistance. The switching frequency is very high for the "critical overload resistance" and lowest for a short circuit. The "critical overload resistance" is defined here as the load resistance slightly lower than the load resistance that barely trips the overcurrent limiter threshold setting.
The high switching frequencies obtained at the critical overload resistance usually exceed the power switching frequency ratings of both the transistor switch and the flyback diode. Thus, it is desirable to provide a power supply having a switching frequency which is held constant and at the highest safe switching rate for all overload resistances including the short circuit as well as the critical load resistance.
In addition, it is desirable to provide a power supply with soft start and soft turn OFF performance characteristics. A soft start may be required in the event that the load requires very high transient starting current. Capacitors, motors, and incandescent lamps are examples of loads requiring high transient starting current. A soft turn OFF may be required in the event that the source of unregulated power is switched off which causes a switching regulator to sense a voltage drop which it tries to compensate for by drawing more current through the switching transistor. In this mode, the unregulated power may be decaying down due to the slow discharge of an input filter capacitor. As a result, the power switch may have enough energy to be biased ON but not enough to be in saturation. In that event very high current may be drawn through the power switch in its active mode. This presents a hazard to a switching transistor without the soft turn OFF feature.